Hydrostatic transmission with overspeed protection

Abstract

Hydrostatic transmission for a drive system, comprising a drive shaft (5) of a primary unit (2) that can be coupled to an internal combustion engine (4) of the drive system and a secondary unit (6) that can be coupled to an output (14) of the drive system, wherein the two units (2, 6) are fluidically connected to each other via two working lines (20, 22) of a closed circuit, and wherein the primary unit (2) has an adjustable swivel angle or an adjustable displacement volume (Vg_pump) that can be controlled by an electrical control unit (40) when the hydrostatic transmission is braked, characterized in that a first engagement threshold (n_eng_on_min) or a first engagement threshold derived therefrom (n_pump_on_min) lies by an additional value above a target speed (n_eng_des) of the internal combustion engine (4) or a value derived therefrom (n_pump_des).and that a further entry threshold or a further entry threshold derived therefrom is provided, wherein the braking can be initiated automatically via the control unit (40) when the first entry threshold (n_eng_on_min) is reached or exceeded by an actual speed (n_eng_act) of the internal combustion engine (4) or by a value derived therefrom (n_pump_act), and when the further entry threshold is also reached or exceeded by a further actual value or by a value derived therefrom.

Description

[0001] The invention relates to a hydrostatic transmission with which hydrostatic braking is possible and which has overspeed protection for an internal combustion engine coupled to the transmission.

[0002] Hydrostatic transmissions for mobile machinery are known from the prior art, in which a hydrostatic pump (primary unit) and a hydrostatic motor (secondary unit) are fluidically connected to each other via a closed circuit. An internal combustion engine, e.g., a diesel engine of the mobile machinery, is coupled to the primary unit, and an output, e.g., an axle or a wheel of the mobile machinery, is coupled to the secondary unit in a rotationally fixed manner. Thus, the mobile machinery has a drive system that incorporates a hydrostatic transmission.

[0003] In publication EP 1 960 699 B1, a hydrostatic transmission is disclosed that can also be used for braking. In this system, the power flows in the opposite direction to traction operation, from the output shaft via the secondary unit (acting as a pump) and the primary unit (acting as a motor) to the internal combustion engine, which is then driven in a passive towing mode. The high-pressure working line of the closed circuit is protected by a pressure relief valve, which also allows a portion of the braking power to be dissipated during braking. However, with this solution, braking is only initiated when the driver explicitly requests it by pressing the brake pedal. Therefore, the driver must also press the brake pedal to prevent excessively high engine speeds.

[0004] German patent applications DE 10 2014 211 393 A1 and US 2014 / 0372000 A1 each disclose a hydrostatic transmission designed to protect the internal combustion engine from overspeed. This involves a speed control system that detects when the engine's braking power is exceeded and automatically initiates high-performance braking. Part of the braking power is transferred to the engine, while another part is dissipated as heat via the pressure relief valve of the affected high-pressure line. This process utilizes a combination of the engine's actual speed and other signals, such as speed and vehicle speed deviations. A disadvantage of this hydrostatic transmission is that there are situations where protecting the engine based solely on the described criteria is insufficient.Furthermore, the gearbox does not take into account the significant differences that may exist between the various mobile work machines affected.

[0005] Furthermore, a hydrostatic transmission is known from the prior art which, when a speed threshold or an engagement threshold of the internal combustion engine's speed is exceeded, triggers high-performance braking by pivoting the primary unit back. While this protects the internal combustion engine, its potential braking power is not optimally utilized.

[0006] Furthermore, patent DE 10 2014 206 123 A1 is known from the prior art.

[0007] In contrast, the invention is based on the objective of creating a hydrostatic transmission in which, in the event of braking, the internal combustion engine is reliably protected from overspeeding, while dissipating or reducing the maximum possible braking power.

[0008] This problem is solved by a hydrostatic transmission with the features of claim 1.

[0009] Further advantageous embodiments of the invention are described in the dependent patent claims.

[0010] The claimed hydrostatic transmission is intended for a traction drive system comprising an internal combustion engine, e.g., a diesel engine, and an output, e.g., a wheel or axle. The hydrostatic transmission has a primary unit's drive shaft, which can be coupled to the traction drive's internal combustion engine and functions as a pump during traction operation, and a secondary unit, or several hydraulically parallel secondary units, which can be coupled to the traction drive's output and function as motor(s) during traction operation. Both units are fluidically connected to each other via two working lines in a closed circuit. The primary unit has an adjustable swivel angle and thus an adjustable displacement, which can be controlled by an electrical control unit during braking of the hydrostatic transmission. The control unit automatically initiates braking when a specific internal combustion engine speed, or a value derived from it, e.g.,The system activates when an actual speed of the primary unit, a first intervention threshold, or a derived intervention threshold (e.g., a primary unit speed) is reached or exceeded, and when a further actual value also reaches or exceeds a second intervention threshold. The first intervention threshold is always higher than a target speed of the internal combustion engine, or a derived value (e.g., a primary unit speed), by a specific allowance or offset. This makes the first intervention threshold variable and always adapted to the current operating situation of the internal combustion engine. The second intervention threshold, and the interaction of the two intervention thresholds, allows the internal combustion engine to be protected flexibly and reliably from overspeeding, while dissipating maximum braking power.Furthermore, the hydrostatic transmission according to the invention can be flexibly adapted to various mobile working machines and to different braking systems.

[0011] If the target engine speed can be adjusted via a driver request, particularly via a control element such as a brake pedal, accelerator pedal, drive lever, or cruise control, the driver's foresight can be used to advantage. For example, the first engagement threshold is indirectly reduced by the driver when they specify a reduction in the target engine speed, and vice versa.

[0012] According to an initial further development, the further entry threshold is a fixed speed of the combustion engine or a fixed entry threshold derived from it.

[0013] According to a second further development, the further entry threshold is a fixed acceleration of the combustion engine or a fixed entry threshold derived from it.

[0014] According to a third further development, the further entry threshold is a variable or adjustable entry threshold of the rotational speed or an entry threshold derived therefrom, which depends on an actual driving speed of the mobile working machine or a value derived therefrom, e.g. an actual rotational speed of the secondary unit.

[0015] A further development approach is particularly preferred in which the three aforementioned intervention thresholds are simultaneously stored or calculated in the control unit. This allows the control unit to automatically initiate braking if – apart from the first intervention threshold – at least one of the following (already mentioned) intervention thresholds is reached or exceeded: - the fixed entry threshold of the internal combustion engine's speed or the derived entry threshold, or -the fixed entry threshold of the acceleration of the internal combustion engine or the entry threshold derived therefrom, or -the variable or adjustable entry threshold of the rotational speed or the derived entry threshold which depends on the actual travel speed of the mobile working machine or the value derived therefrom.

[0016] The further variable or adjustable entry threshold of the rotational speed or the entry threshold derived therefrom is preferably less than or equal to the further fixed entry threshold of the rotational speed or the entry threshold derived therefrom.

[0017] The further variable or adjustable entry threshold of the speed or the entry threshold derived therefrom is set by the control unit to be equal to the further fixed entry threshold of the speed or the entry threshold derived therefrom if the actual travel speed of the mobile working machine or the value derived therefrom is less than or equal to a target travel speed or a value derived therefrom, and it is set to be less than the further fixed entry threshold of the speed if the actual travel speed or the value derived therefrom is greater than the target travel speed of the mobile working machine or the value derived therefrom.

[0018] It is preferred if the braking is also automatically initiated by the control unit when the actual speed of the combustion engine, or a value derived from it, drops below a speed threshold or a derived threshold, e.g., a speed threshold of the primary unit. The threshold may depend on the mobile machine.

[0019] The derived entry thresholds and values ​​frequently mentioned in this document are, in particular, equal to or proportional to the respective previously mentioned entry threshold or value. This is possible, for example, if the two affected entry thresholds of a given rotational speed are coupled via a single shaft, or if they are rotationally fixed to each other via a mechanical transmission. Thus, it is possible, for example, to derive the rotational speed thresholds of the crankshaft of the internal combustion engine (not belonging to the transmission under consideration) from the drive shaft of the primary unit (of the transmission under consideration) coupled to it.

[0020] To enable high-performance braking, it is particularly preferred to have a pressure relief valve on each of the two working lines. A first portion of the braking force can be dissipated via the pressure relief valve affected by the braking action, while a second portion can be dissipated via the primary unit and the internal combustion engine. The achievable braking force is particularly high when the first portion is greater than the second.

[0021] During high-performance braking, if the flow rate through the primary unit increases, the flow rate through the affected pressure relief valve decreases. This can cause the pressure in the high-pressure working line to drop. To minimize this pressure reduction or to keep the pressure nearly constant, pressure relief valves with a flat characteristic curve regarding their pressure differential as a function of the flow rate are preferred.

[0022] In a preferred embodiment of the hydrostatic transmission according to the invention, the swivel angle and the stroke volume of the primary unit are adjustable on both sides of a zero position. This allows the affected drive system to be used in both directions of travel of the mobile work machine in towing operation, while maintaining the same direction of rotation of the combustion engine, and to be braked accordingly in both directions (high-performance) according to the invention.

[0023] If the secondary unit also has an adjustable swivel angle and thus an adjustable stroke volume, which can be controlled by the control unit during braking, then a braking torque can be adjusted during braking.

[0024] Preferably, the braking torque during braking is variable or adjustable, particularly depending on the control element, i.e., the driver's request.

[0025] An embodiment of the transmission according to the invention is shown in the drawings. The invention will now be explained in more detail with reference to the figures in these drawings.

[0026] They show Fig. 1 a hydraulic and electrical circuit diagram of a drive system with a hydrostatic transmission according to the invention as described in the exemplary embodiment, Fig. 2 a diagram of the different engagement thresholds of the gearbox Fig. 1, Fig. 3 a diagram of the acceleration of the internal combustion engine and thus of the primary unit of the hydrostatic transmission from Fig. 1 and Fig. 4 a diagram of the rotational speeds of the internal combustion engine and thus of the primary unit of the hydrostatic transmission Fig. 1.

[0027] According to Fig. In the invention, a hydrostatic drive system of a (not shown) mobile working machine (e.g., wheel loader, telehandler, combine harvester, or forage harvester) comprises a hydrostatic transmission 1. The transmission 1 has a hydrostatic primary unit 2, which is primarily operated as a hydraulic pump and is driven by an internal combustion engine 4 of the drive system, designed as a diesel engine, via a drive shaft 5. Furthermore, the transmission 1 has a hydrostatic secondary unit 6, which is coupled via a drive shaft 8 to an axle 14 of the drive system having two wheels 12, and which is primarily operated as a hydraulic motor. More precisely, the drive shaft 8 is coupled to a differential gear 10 of the axle 14.

[0028] Both hydraulic machines 2, 6 have adjustable stroke volumes Vg_pump and Vg_mot via respective adjusting devices 16 and 18. The first hydraulic machine 2 is fluidically connected to the secondary unit 6 in a closed hydraulic circuit via a first working line 20, which is the supply line in the following considerations and through which hydraulic fluid flows from the primary unit 2 to the secondary unit 6, and via a second working line 22, which is the return line in the following considerations and through which hydraulic fluid flows from the secondary unit 6 to the primary unit 2.

[0029] The hydrostatic transmission 1 has a feed pump 26 connected to the drive shaft 5 of the primary unit 2, which can pump hydraulic fluid from a tank T into a feed line 28. The latter branches into three branches, the first of which can be connected to the tank T via a pressure relief valve 30. A second and a third branch can be connected to branch line 20 and branch line 22, respectively, via pressure relief valves 32 and 34, each of which has an integrated suction check valve 36 and 38.

[0030] Both units 2, 6 can be operated in all four quadrants, so that both the flow direction of the pressure medium in the closed hydraulic circuit and the direction of rotation of each of the units 2, 6 are reversible.

[0031] The hydrostatic transmission 1 has a control unit 40 to which a brake pedal 44 is connected via a signal line 42. The brake pedal 44 has a sensor 46 which detects the force applied to the brake pedal 44 and transmits this information to the control unit 40 via the signal line 42. The control unit 40 is connected via an electrical signal line 48 to the adjusting device 16 of the primary unit 2 and via an electrical signal line 50 to the adjusting device 18 of the secondary unit 6.

[0032] An electrical signal line 52 connects a speed sensing unit 54, which detects the actual speed n_mot_act of the secondary unit 6 at the drive shaft 8, to the control unit 40. An electrical signal line 62 connects a speed sensing unit 60, which detects the actual speed n_pump_act of the primary unit 2 at its drive shaft 5, to the control unit 40. Due to the integral design of the drive shaft 5 with a crankshaft of the internal combustion engine 4, which is protected against overspeed, the speed sensing unit 60 also detects the actual speed n_eng_act of the internal combustion engine 4, which is to be limited.

[0033] In high-performance braking using the hydrostatic transmission 1 according to the invention, the axle 14 is supported via the drive shaft 8 and via the secondary unit 6, which acts as a pump, and via one of the two working lines 22, and via the primary unit 2, which acts as a motor, and via the drive shaft 5 of the primary unit 2 on the internal combustion engine 4, which is then dragged along and, via its friction and acceleration forces of the pistons, dissipates at least part of the braking energy of the mobile working machine.

[0034] Furthermore, a cruise control 64, an accelerator pedal 66 and a drive lever 68 are electrically connected to the control unit 40 via respective signal lines.

[0035] During operation of the hydrostatic transmission 1 according to the invention, the control unit 40 calculates a target rotational speed n_mot_des of the drive shaft 8 of the secondary unit 2 from the setting of the cruise control 64 or the position of the accelerator pedal 66 or the drive lever 68, all of which represent a driving request, since this is proportional to the target travel speed v_veh_des of the mobile working machine in question. The actual travel speed v_veh_act is then derived accordingly from the actual rotational speed n_mot_act of the secondary unit 6.

[0036] The control unit 40 has a memory unit 56 in which two fixed entry thresholds n_eng_on, α_eng_on are stored, and a processor unit 58 in which two variable entry thresholds n_eng_on_min, n_eng_on_v_veh are calculated and from which the high-performance braking is automatically initiated and executed depending on all four entry thresholds n_eng_on_min, n_eng_on, n_eng_on_v_veh, α_eng_on

[0037] Fig. Figure 2 shows the interaction of the four different entry thresholds n_eng_on_min, n_eng_on, n_eng_on_v_veh, α_eng_on. In Fig. Figure 2 below shows the first activation threshold n_eng_on_min, which the actual speed n_eng_act of the combustion engine 4 must exceed for the high-performance braking to be triggered automatically. The first activation threshold n_eng_on_min is always higher than the target speed n_eng_des by a specific allowance value or offset.

[0038] In Fig. Figure 2 also shows the three further activation thresholds n_eng_on, n_eng_on_v_veh, α_eng_on, one of which must be exceeded for the high-performance braking to be triggered. More precisely, beyond the first activation threshold n_eng_on_min mentioned above, either the actual rotational speed n_eng_act of the crankshaft of the internal combustion engine 4 must exceed the fixed speed threshold n_eng_on or the variable speed threshold n_eng_on_v_veh, or the acceleration α_eng_act of the crankshaft of the internal combustion engine 4 must exceed the acceleration threshold α_eng_on. The variable speed threshold n_eng_on_v_veh is calculated as a function of the actual travel speed v_veh_act and the target travel speed v_veh_des of the mobile machine in question, as follows: v_veh_act≤v_veh_deh→n_eng_on_v_veh=n_eng_on v_veh_act>v_veh_deh→n_eng_on_v_veh=0.9⋅n_eng_on

[0039] The further intervention threshold n_eng_on_min has the task of preventing high-performance braking in the following two cases, when it is not yet needed to protect the combustion engine from overspeeding: 1.) During road driving at a relatively low actual engine speed n_eng_act, where the combustion engine 4 still has sufficient speed reserve. The aim is to prevent high-performance braking from being triggered during high actual acceleration α_eng_act of the combustion engine 4 (α_eng_act > α_eng_on), even though the actual engine speeds n_eng_act are not critical. 2.) During operation, the combustion engine 4 of, for example, a combine harvester or forage harvester operates at very high constant speeds n_eng_act. High-performance braking is not required because the actual driving speed v_veh_act is relatively low, and because the combustion engine 4 is additionally loaded by consumers such as the threshing unit, intake, and grain cutter. When these consumers are switched off, the combustion engine 4 is relieved of load, causing the actual speed n_eng_act to briefly increase slightly until it is reduced again by a speed controller. Since these high constant speeds n_eng_act are already very close to the additional activation threshold n_eng_on, it is possible that this threshold will be exceeded during the load reduction, even though high-performance braking is not required. For this reason, the additional activation threshold n_eng_on_min is variable and has a fixed value or offset relative to the target speed n_eng_des.

[0040] In an initial operating situation, the target speed n_eng_des is 1500 rpm, the allowance value is 150 rpm, so the first activation threshold n_eng_on_min is 1650 rpm. The second activation threshold n_eng_on is 2100 rpm and thus higher than the first. Therefore, the high-performance braking is only triggered when the second activation threshold n_eng_on of 2100 rpm is exceeded.

[0041] In a second operating scenario, the target speed n_eng_des is 2000 rpm. With an input value of 150 rpm, the first activation threshold n_eng_on_min is 2150 rpm. Therefore, the subsequent activation threshold n_eng_on, at 2100 rpm, is slightly lower than the first. Thus, high-performance braking is only triggered when the first activation threshold n_eng_on_min of 2150 rpm is exceeded.

[0042] This example shows that the first entry threshold n_eng_on_min (despite its name or reference symbol) is not always the smaller entry threshold.

[0043] Fig. 3 and Fig. Figure 4 shows further examples of the entry and exit thresholds in a chronological sequence. First, an entry, i.e., the automatic initiation of high-performance braking, is shown, followed by an exit, i.e., the automatic termination of high-performance braking.

[0044] Fig. Figure 3 shows the further fixed acceleration threshold α_eng_on and an exemplary progression of the actual acceleration α_eng_act of the crankshaft of the internal combustion engine. Figure 4 shows that when the actual rotational speed n_eng_act exceeds the first acceleration threshold n_eng_on_min (see Figure 3), the following occurs: Fig. 4) and according to Fig. 3. If the actual acceleration α_eng_act exceeds the entry threshold α_eng_on, high-performance braking is triggered.

[0045] Fig. Figure 4 shows the various rotational speeds n_eng in relation to each other. In addition to the target rotational speed n_eng_des, an example of the actual rotational speed n_eng_act is shown, along with the different speed thresholds. More precisely, the first fixed activation threshold n_eng_on_min, the second variable activation threshold n_eng_on_v_veh, the third fixed activation threshold n_eng_on, and the final activation threshold n_eng_off are shown. It can be seen that high-performance braking is triggered after exceeding the fixed activation threshold n_eng_on_min and additionally upon reaching the second variable activation threshold n_eng_on_v_veh, and continues until the actual rotational speed n_eng_act drops to the final activation threshold n_eng_off.

[0046] A hydrostatic transmission for a drive system is disclosed, in which a variable displacement pump and one or more motors are coupled together in a closed hydraulic circuit. An electronic control unit can initiate braking via the transmission into the drive system if the internal combustion engine is at risk of overspeeding. Braking is initiated when at least two engagement thresholds are exceeded, the first of which is variable, while the second is either fixed or variable. Reference symbol list 1 hydrostatic transmission 2 Primary unit 4 Internal combustion engine 5 Drive shaft 6 Secondary unit 8 Drive shaft 10 Differential gears 12 wheel 14 Output / Axle 16 Adjustment device 18 Adjustment device 20 Work management 22 Work management 26 Feed pump 28 Feed line 30 Pressure relief valve 32 Pressure relief valve 34 Pressure relief valve 36 Suction check valve 38 Suction check valve 40 Control unit 42 Signal line 44 Brake pedal 46 Sensor 48 Signal line 50 Signal line 52 Signal line 54 Speed ​​detection unit 56 storage units 58 processor units 60 speed detection unit 62 Signal line 64 Cruise control 66 Accelerator pedal 68 Driving levers n_eng_act Actual speed of the internal combustion engine n_eng_des target speed of the combustion engine n_eng_off Exit threshold (engine speed) n_eng_on further entry threshold (speed of the internal combustion engine) n_eng_on_min first entry threshold (speed of the internal combustion engine) n_eng_on_v_veh further entry threshold (speed of the internal combustion engine) n_mot_act Actual rotational speed of the secondary unit n_pump_act Actual rotational speed of the primary unit n_pump_des target speed of the primary unit n_pump_off Exit threshold (rotational speed of the primary unit) n_pump_on derived further entry threshold (rotational speed of the primary unit) n_pump_on_min derived first entry threshold (rotational speed of the primary unit) n_pump_on_v_veh derived further entry threshold (rotational speed of the primary unit) Vg_mot secondary unit displacement Vg_pump Primary unit displacement volume v_veh_act Actual driving speed v_veh_des target speed α_eng_act Actual acceleration of the combustion engine α_eng_on further entry threshold (acceleration of the combustion engine) α_pump_on further entry threshold (acceleration of the primary unit) T Tank

Claims

[1] Hydrostatic transmission for a drive system, comprising a drive shaft (5) of a primary unit (2) that can be coupled to an internal combustion engine (4) of the drive system and a secondary unit (6) that can be coupled to an output (14) of the drive system, wherein the two units (2, 6) are fluidically connected to each other via two working lines (20, 22) of a closed circuit, and wherein the primary unit (2) has an adjustable swivel angle or an adjustable displacement volume (Vg_pump) that can be controlled by an electrical control unit (40) when the hydrostatic transmission is braked, characterized by, that a first entry threshold (n_eng_on_min) or a first entry threshold derived therefrom (n_pump_on_min) lies by an additional value above a target speed (n_eng_des) of the internal combustion engine (4) or a value derived therefrom (n_pump_des), and that a further entry threshold or a further entry threshold derived therefrom is provided, wherein braking can be initiated automatically via the control unit (40) if the first entry threshold (n_eng_on_min) is reached or exceeded by an actual speed (n_eng_act) of the internal combustion engine (4) or by a value derived therefrom (n_pump_act), and if the further entry threshold is also reached or exceeded by a further actual value or a value derived therefrom. [2] Hydrostatic transmission according to claim 1, wherein the target speed (n_eng_des) or the value derived therefrom (n_pump_des) is adjustable via a control element. [3] Hydrostatic transmission according to claim 1 or 2, wherein the further engagement threshold is fixed. [4] Hydrostatic transmission according to claim 3, wherein the further fixed engagement threshold is a rotational speed (n_eng_on) of the internal combustion engine (4) or a fixed engagement threshold derived therefrom (n_pump_on). [5] Hydrostatic transmission according to claim 3, wherein the further fixed entry threshold is an acceleration (α_eng_on) of the internal combustion engine (4) or a fixed entry threshold derived therefrom (α_pump_on). [6] Hydrostatic transmission according to claim 1 or 2, wherein the further engagement threshold is a variable or adjustable engagement threshold of the rotational speed (n_eng_on_v_veh) or an engagement threshold derived therefrom (n_pump_on_v_veh) which depends on an actual driving speed (v_veh_act) of the mobile working machine or a value derived therefrom (n_mot_act). [7] Hydrostatic transmission according to claim 1 or 2, wherein the following three further entry thresholds are simultaneously stored or calculable in the control unit (40), and wherein the braking can be initiated automatically by the control unit (40) if at least one of these entry thresholds is reached or exceeded: - a fixed entry threshold of the rotational speed (n_eng_on) of the internal combustion engine (4) or an entry threshold derived therefrom (n_pump_on), or -a fixed entry threshold of the acceleration (α_eng_on) of the internal combustion engine (4) or an entry threshold derived therefrom (α_pump_on), or -a variable or adjustable entry threshold of the rotational speed (n_eng_on_v_veh) or a derived entry threshold (n_pump_on_v_veh) which depends on an actual travel speed (v_veh_act) of the mobile working machine or a derived value (n_mot_act). [8] Hydrostatic transmission according to claim 7, wherein the further variable or adjustable entry threshold of the rotational speed (n_eng_on_v_veh) or the entry threshold derived therefrom (n_pump_on_v_veh) is less than or equal to the further fixed entry threshold of the rotational speed (n_eng_on) or the entry threshold derived therefrom (n_pump_on). [9] Hydrostatic transmission according to claim 8, wherein the further variable or adjustable engagement threshold of the rotational speed (n_eng_on_v_veh) or the engagement threshold derived therefrom (n_pump_on_v_veh) is set equal to the further fixed engagement threshold of the rotational speed (n_eng_on) or the engagement threshold derived therefrom (n_pump_on) when the actual travel speed (v_veh_act) of the mobile working machine or the value derived therefrom (n_mot_act) is less than or equal to a target travel speed (v_veh_des) of the mobile working machine or a value derived therefrom (n_mot_des), and wherein the further variable or adjustable engagement threshold of the rotational speed (n_eng_on_v_veh) or the engagement threshold derived therefrom (n_pump_on_v_veh) is set lower than the further fixed engagement threshold of the rotational speed (n_eng_on) or the engagement threshold derived therefrom (n_pump_on),if the actual driving speed (v_veh_act) of the mobile working machine or the value derived from it (n_mot_act) is greater than the target driving speed (v_veh_des) of the mobile working machine or the value derived from it (n_mot_des). [10] Hydrostatic transmission according to one of the preceding claims, wherein the braking can be automatically deactivated when the actual rotational speed (n_eng_act) of the internal combustion engine (4) or the value derived therefrom (n_pump_act) drops below a speed threshold (n_eng_off) or a derived speed threshold (n_pump_off). [11] Hydrostatic transmission according to one of the preceding claims, wherein a pressure relief valve (32, 34) is arranged on each of the two working lines (20, 22), and wherein the braking is a high-performance braking system in which a first part of the braking power can be reduced via one of the pressure relief valves (32, 34), while a second part of the braking power can be reduced via the primary unit (2). [12] Hydrostatic transmission according to claim 10, wherein the pressure limiting valves (32, 34) each have a flat characteristic curve with respect to their pressure difference as a function of their volume flow rate. [13] Hydrostatic transmission according to one of the preceding claims, wherein the swivel angle and the stroke volume (Vg_pump) of the primary unit (2) are adjustable on both sides of a zero position. [14] Hydrostatic transmission according to one of the preceding claims, wherein the secondary unit (6) has an adjustable swivel angle or an adjustable stroke volume (Vg_mot) which is controllable by the control unit (40) during braking. [15] Hydrostatic transmission according to one of the preceding claims, wherein a braking torque is variable or adjustable during braking.

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